This invention relates to the protection of radiation receiving devices from damage by an incident beam of radiation of excessive intensity and, more particularly, to a protection system employing a chamber of gaseous material with raised energy states, and located on a path of radiation propagation, for absorption of excess incident radiation power.
Radiation receiving devices are employed in a variety of situations. Of particular interest are receiving devices employed in the imaging of scenes emitting radiation such as infrared radiation. An infrared imaging system employs, typically, an array of radiation sensitive detectors which view incoming radiation via an optical focusing system including a scanning mirror. The detectors output signals in response to the incident radiation, the detector signals being employed by well-known signal processing circuitry to produce an image of the scene upon a display for viewing by persons operating the infrared imaging system.
Radiation detectors, of the type employed in an array of detectors may be fabricated of semiconductor material which is susceptible to damage, as by heating, when exposed to excessively strong radiation. For example, a laser beam which might be directed inadvertently or deliberately toward the receiving optics of an imaging system could inflict significant damage to the radiation detectors so as to disable the detectors.
Such detectors, or sensors of radiation, are found in laser rangefinders, radars, and passive receiving systems such as the aforementioned imaging system. The receiving systems employ receiving telescopes which may have a relatively large field of view and a large acceptance angle through which laser radiation may be received. In the case of an active device, such as a laser rangefinder, it is intended that the receiving telescope receive laser radiation. However, in the case of a passive infrared imaging system, it is intended that only scene radiation be incident upon the receiving telescope even though the telescope is responsive to laser radiation at the infrared frequencies.
The laser radiation can be at a variety of wavelengths, and may be pulsed or continuous wave. In order to protect a receiving device from excessively strong laser radiation, a fast-operating optical switch is desired to close the optical path in the presence of the strong radiation, which switch must allow the normal intensity radiation to pass without interference through the optical system to the detectors. Such an optical switch must be activated almost instantaneously, prior to the damaging of the detectors, to block or greatly reduce the transmission of strong laser radiation. Ideally, such an optical switch should protect against a broad band of laser wavelengths, and be operative over a wide field of view.
Detectors or sensors of infrared radiation utilized in imaging systems operate typically over narrow bands of radiation, on the order of several microns, and are susceptible to damage by high energy radiation such as that emitted by lasers operating within the receiving band of the detectors. The use of conventional fixed absorbing or reflecting filters with relatively large effective bandwidths is usually precluded in the case of multiline incident laser radiation. The ineffective protection offered by the filters results because protection against all possible laser wavelengths would require the use of an excessively large number of filters which would reduce excessively the sensor radiation passband.
One attempt to provide suitable protection of an array of detectors against excessively powerful radiation has been the use of a resonant gaseous absorption chamber disposed along the path of radiation propagation. As an example of the gas absorption process, pulse shaping has been employed with radiation from a carbon dioxide laser by passage of the radiation through a saturable absorber gaseous material such as sulphur hexafluoride.
However, a problem exists in that such saturable absorbing material has not been adequately effective in reducing a strong beam of radiation to extinction. The problem arises because presently available devices do not produce enough numerical density of molecules in a resonant state for interaction with the incident radiation. Therefore, presently available resonant-gas absorbers do not provide adequate protection against intense radiation, and are limited to specific wavelengths (usually one wavelength) wherein threats of damaging radiation may or may not operate.